in physical combat I think the Jem'Hadar win it easily. They're genetically bred for it, while the Borg's technical additions give them a lot of vulnerabilities on their body. Also, the Borg collective doesn't value individual drones, so there's no reason to make them great fighters.

On a larger level though, the Borg win for their superior technology and ability to adapt.

I think the Borg would eventually win, but it'd take a very long time, and the founders would get away. But they'd probably send only one cube at a time, and they'd probably be able to handle the first cube.

I can imagine, in the long run, the founders just kinda sacrificing all their subjects and most of their Gem'Hadar and just escaping to another galaxy.

Hmm --- I had always assumed the Borg encountered Dominion ships here and there probably assimilated them, but never gave much thought to the actual battles.

Here's some random musings on the topic:

The big reason the Federation had problems with the Dominion wasn't so much because they were technologically superior, its because they had technology that worked differently and it's differences weren't something the Federation had had to deal with up until that point.

In the Alpha and Beta Quadrant most species ended up bartering their technology to some degree so most technology evolved along similar paths. The Dominion's technology was generally able to over load the Federation and other Alpha/Beta quadrant species because of the incompatibilities. Once the Federation learned how to adapt, it was certainly a much more level playing field.

That being said, one would have to believe the Borg would be able to adapt much more easily (since that's what they do!). I'd honestly anticipate that if the Borg sent 1 cube at a time at the Dominion, they'd be able to repel it like the Federation is able to. Likewise, if they sent four or five at them at the same time, it might get more then a little hard to handle.

On the personal combat end of things --- I think that becomes a far more interesting question. We've heard references to "Tactical Drones" and "X species make really good Tactical drones", but I think it can be argued we've never actually seen tactical drones and what they're capable of. Do they exclusively use energy based weapons or has the Borg invested in making physically strong drones capable of hand to hand combat? I think its really hard to rule out that they don't have something like this. I mean every "hive" has its "warrior class" or "defender class". Overall, I'd be inclined to believe we don't really have enough information in canon-esque areas to know one way or the other how the Gem'hedar would equit themselves in a true physical brawl with the borg.

I'd say the concept of Borg evolved a ton since anything in TNG (minus First Contact).

I don't really envision anything like Predator, but I'd expect if someone with some Trek authority would design a "tactical drone" today (or even during the Voyager era) --- it would have been something more fitting of of a "borg warrior class".